The invention relates to a method of producing a conductor composed of niobium and copper and, if required, other electrically normal-conducting metals. The method is especially applicable for producing conductors for superconducting cables.
In the manufacture of various superconducting components, such as superconducting cables for the transmission of large amounts of electric energy, particularly for single-phase and three-phase alternating current, or band-shaped alternating current superconductors, it is often desirable to bond a superconductive layer of niobium to a metal which has a good thermal conductivity and which has good normal-conducting characteristics at the low temperature required for maintaining the superconductivity of the niobium. Niobium is a particularly well suited superconductor for single-phase and three-phase cables because it has a very high, below critical magnetic field H.sub.cl of about 120,000 A/m. Thus it appears advantageous for superconducting single-phase or three-phase cables to arrange, for example, copper or aluminum tubes coaxially with respect to each other, the tubes being provided on the outside or inside with a layer of niobium. Preferably, the niobium layer is on the outside of the inner tube and on the inside of the outer tube. By making the inner tube the outgoing conductor and the outer tube as the return conductor, the electric and magnetic fields occur only in the space between the niobium layers, and the tubes of electrically normal-conducting metal remain free of fields, so that no eddy current losses can occur therein.
The normal-conducting metal serves here particularly to electrically stabilize the superconducting niobium by taking over at least partially the currents flowing in the superconducting niobium if the niobium passes from the superconducting to the electrically normal-conducting state, for example, as can happen in the event of an overload. The normal-conducting metal also diverts the heat loss generated in the niobium in such instances or by alternating-current losses in the niobium to an adjoining cooling medium which is preferably liquid helium. For this purpose, the closest possible electrically and thermally good conducting contact between the niobium and the electrically normal-conducting metal is required.
A close, highly adhesive bond of niobium with highly conductive metals such as copper, however, presents considerable difficulties and cannot be achieved, for example, by simply soldering copper to niobium or by electrolytic deposition of copper on niobium. The known method of coating copper with niobium by electrolytic deposition of the niobium in a melt on a copper carrier are not applicable in cases where a prefabricated niobium part, for example, a niobium foil, is to be connected with the normal-conducting metal such as copper. This is often desirable for manufacturing reasons, for example, if a prefabricated niobium foil is to be used as the superconductor, and is necessary particularly if the niobium is to be subjected to pretreatments for reducing the alternating current losses, for example, annealing for several hours at temperatures of 2000.degree.C or more in an ultra-high vacuum. Because of the low melting temperature of copper, such an annealing treatment of a niobium layer already deposited on a copper carrier is not possible, because already at temperatures near the melting point of the copper, deformations of the copper carrier occur due to creep processes and are intolerable for later applications. In such a case, the niobium must therefore be first annealed alone and can be bonded to the copper only after the annealing. Also in other cases it may be necessary for manufacturing reasons to prefabricate the niobium parts, and to only then bond these parts to copper.
In the U.S. patent application Ser. No. 23,359, filed on Mar. 27, 1970, now U.S. Pat. No. 3,703,447, it is suggested that for coating niobium with copper, the metallically clean niobium surface to be coated be brought into contact with copper which is metallically clean at least on the surface and to then bond the copper to the niobium by subsequently heating the niobium to a temperature of between 800.degree. and 2500.degree.C in a vacuum with a residual gas pressure of at most 10.sup.-.sup.6 Torr.
Although this method yields an intimate bond between niobium and copper, it is expensive because of the high temperatures involved. At these high temperatures there furthermore exists the danger that the niobium can absorb such residual gases as oxygen and hydrogen even in a high vacuum because of its high reactivity with respect to these gases. Since, especially in the case of alternating current superconductors, the current is transported only in a thin surface layer, such gas reactions are undesirable because of an increase in the alternating current losses associated therewith.